At low magnetic field, for example at 1.5 Tesla, the magnetic resonance frequency is relatively low, resulting in a corresponding radio frequency (RF) wavelength that is relatively long compared to the size of the object. As a result, a low magnetic field tends to provide good spatial uniformity on the scale of a typical human anatomical region of interest (e.g., head, torso, limb, or so forth). Accordingly, those skilled in the art have typically employed a volume coil such as a birdcage coil for magnetic resonance excitation, and have used a volume coil or a local coil (possibly comprising a plurality of coil elements) for magnetic resonance signal reception. The tendency toward good spatial uniformity for magnetic resonance at low magnetic field has led to substantial clinical and diagnostic success with such systems. However, low magnetic field has certain disadvantages with acquisition time constraints in mind, such as relatively lower spatial resolution, lower signal strength and correspondingly lower signal-to-noise ratio (SNR), and so forth.
Accordingly, there has been continued interest in performing magnetic resonance imaging and spectroscopy at higher magnetic fields. At high magnetic field, for example at 7 Tesla, the magnetic resonance frequency is substantially higher (in proportion to the main field), the corresponding radio frequency (RF) wavelength is substantially shorter, and tissue properties and object shape (load) can lead to substantial B1 non-uniformity on the scale of a typical human head or extremity such as calf. As a result, at high magnetic field those skilled in the art have explored ways to improve B1 uniformity for head imaging and use local coils for other anatomical regions for both magnetic resonance excitation and reception.
At 1.5 Tesla, if the same surface coil (which, again, may be an array or other plurality of coil elements) is used for both excitation and reception, it has generally been accepted that the coil will excite and read from the same region of sensitivity of the proximate load. This is because the |B1+| transmit field and the |B1−| field sensed by the coil are both reasonably similar, having regions of sensitivity that substantially overlap. At higher fields like 7 T, the region of sensitivity may exhibit substantial load-induced non-uniformity and asymmetry, even for a uniform object. Even worse, the transmit and receive sensitivity patterns are different spatially. Sensitivity in this context refers to the |B1+| field that is generated at a spatial point per unit current and the |B1−| spatial field intensity that can generate a unit of current in the receive antenna. Recognition of the erroneous presumption that the transmit and receive fields substantially overlap has led to certain improvements in the art as disclosed herein.